Heat treatment of cold rolled steel strip

Abstract

A method of heat treating a high strength cold rolled steel strip including a) soaking a cold rolled steel strip, b) cooling the soaked steel strip c) heat treating the cooled strip; d) cooling the heat treated steel strip to ambient temperature range; such that the steel strip has a microstructure including various ferrites, retained austenite and martensite. The main components in the steel composition includes carbon, manganese, silicon and aluminium in addition to iron.

The invention claimed is: 
     
         1 . A method of heat treating a cold rolled steel strip, which method comprises the steps of:
 a) heating an uncoated cold rolled steel strip from a first temperature, wherein the first temperature is room temperature, to a temperature T 1  in the range of 680-740° C. at a single constant heating rate V 1  of 10.0-30.0° C./s;   and further heating the uncoated cold rolled steel strip from the temperature T 1  to a soaking temperature T 2  within a soaking temperature range of (Ac3−20)-(Ac3+20) at a heating rate V 2  of 0.5-4.0° C./s,   a1) then soaking the uncoated cold rolled steel strip at the soaking temperature T 2  within the temperature range of (Ac3−20) to (Ac3+20) for a soaking time t 2  of 30-150 seconds, thereby obtaining a cold rolled steel strip having an austenitic microstructure;   b) cooling the uncoated soaked steel strip resulting from step a1) to a temperature T 4  in the range of Bn-Ms, wherein acicular ferrite is formed during the cooling in a temperature between Bs and Ms; wherein step b) comprises a substep of cooling the soaked steel strip from step a) to a temperature T 3  in the range of 750-600° C., at a cooling rate V 3  of 2.0-15.0° C./s;   and further comprising a substep of cooling the soaked steel strip from a temperature T 3  to T 4  at a cooling rate V 4  of 20.0-60.0° C./s;   c) increasing temperature of the uncoated cooled strip obtained in step b) to a temperature T 5  in a temperature range between Bs and T 4 , to transform austenite to carbide-free LBF and   c1) heat treating the uncoated cooled strip obtained in step c) at the temperature T 5  in a range between the temperature Bs and T 4  for a period of time t 5  of 30-300 seconds;   c2) following heat treatment of step c1) optionally providing the heated treated steel strip with a coating,   d) cooling the heat treated, optionally coated, steel strip to a temperature T 7  in the range of ((Ms−50) to Mf) at a cooling rate of V 7  of 5.0-10.0° C./s and further cooling the heat treated, optionally coated, steel strip to ambient temperature at a cooling rate of V 8  of 5.0-20.0° C./s;   such that the heat treated, optionally coated, steel strip has a microstructure (in vol. %) comprising   polygonal ferrite (PF): 0-10;   polygonal ferrite (PF)+acicular ferrite (AF)+carbide-free higher bainitic ferrite (HBF): 5-30;   carbide-free lower bainitic ferrite (LBF): 45-80;   retained austenite (RA): 5-20;   fresh martensite (M): 0-20;   wherein carbide-free higher bainitic ferrite (HBF) is present;   wherein the steel strip has a composition (in mass percent) comprising   C: 0.15-0.28;   Mn: 1.70-3.00;   Si: 0.50-2.00;   Al: 0.01-0.60;   P: less than 0.050;   S: less than 0.020;   N: less than 0.0080;   wherein the sum of (Si+Al) is ≥0.60; and   wherein 10C+Mn+Cr≥3.85 and 8.5≤(Mn+Cr)/C≤16; and   optionally one or more elements selected from   0<Cr≤0.35;   0<Cu≤0.20;   0<Ni≤0.50;   0<Mo≤0.30;   0<Nb≤0.10;   0<V≤0.10;   0<Ti≤0.10;   0<B≤0.0030;   0<Ca≤0.0050;   0<REM≤0.0100, wherein REM is one or more rare earth metals;   and the remainder being iron and inevitable impurities.   
     
     
         2 . The method according to  claim 1 , wherein the microstructure comprises 52-80 vol. % carbide-free lower bainitic ferrite (LBF). 
     
     
         3 . The method according to  claim 1  wherein step b) comprises cooling the soaked steel strip from step a) to the temperature T 4  at a cooling rate sufficient to avoid pearlite formation. 
     
     
         4 . The method according to  claim 1 , wherein step b) comprises a substep of cooling the soaked steel strip from a temperature T 3  in the range of 800-550° C., to T 4  at a cooling rate V 4  of at least 15° C./s. 
     
     
         5 . The method according to  claim 1 , wherein the microstructure of the steel is carbide-free. 
     
     
         6 . The method according to  claim 1 , wherein step c) is performed at least partially by latent heat produced by the bainite transformation. 
     
     
         7 . The method according to  claim 1 , wherein the heat treating step c) is performed in the range of Bn-(Ms+50). 
     
     
         8 . The method according to  claim 1 , comprising a further heat treatment step between steps c1) and d) of heating the uncoated steel strip resulting from step c1) from the temperature T 5  to a temperature T 6  in the range of Bs-Bn. 
     
     
         9 . The method according to  claim 1 , comprising a further heat treatment step between steps c1) and d) of heating the steel strip resulting from step c1) from the temperature T 5  to a temperature T 6  in the range of Bs-Bn, wherein the further heat treatment step comprises a hot dip galvanizing treatment. 
     
     
         10 . The method according to  claim 1 , following heat treatment of step c1) further comprising a coating step of coating the heated treated steel strip with a protective coating. 
     
     
         11 . The method according to  claim 1 , wherein the microstructure comprises in vol. %:
 polygonal ferrite (PF) 0-5;   polygonal ferrite (PF)+acicular ferrite (AF)+carbide-free higher bainitic ferrite (HBF): 10-25;   lower bainitic ferrite (LBF): 50-75;   retained austenite (RA): 7-15;   fresh martensite (M): 0-15;   and/or wherein the C content in retained austenite (RA) is 0.90 wt. % or more.   
     
     
         12 . The method according to  claim 1 , wherein the resulting steel strip has at least one of the properties:
 Yield strength (YS) is at least 550 MPa; and/or   Tensile strength (TS) is at least 980 MPa; and/or   Total elongation (TE) is at least 13%; and/or   Hole expansion capacity (HEC) is at least 20%; and/or   Bending angle (BA) is at least 80°.   
     
     
         13 . The method according to  claim 1 , wherein polygonal ferrite (PF): 4-5, and
 polygonal ferrite (PF)+acicular ferrite (AF)+carbide-free higher bainitic ferrite (HBF): 19-23.   
     
     
         14 . The method according to  claim 13 , wherein tensile strength is 1127-1153 MPa. 
     
     
         15 . The method according to  claim 14 , wherein HEC=>25%. 
     
     
         16 . The method according to  claim 15 , wherein UE=>10.5% and TE=>14.9%. 
     
     
         17 . The method according to  claim 1 , wherein step b) comprises a substep of cooling the soaked steel strip from step a) to a temperature T 3  in the range of 750-600° C., at a cooling rate V 3  of 3.0-10.0° C./s. 
     
     
         18 . A heat treated cold rolled steel strip made by the method of  claim 1  and having a composition (in mass %) comprising:
 C: 0.15-0.28; 
 Mn: 1.70-3.00; 
 Si: 0.50-2.00; 
 Al: 0.01-0.60; 
 P: less than 0.050; 
 S: less than 0.020; 
 N: less than 0.0080; 
 wherein the sum of (Si+Al) is ≥0.60; and 
 wherein 10C+Mn+Cr≥3.85 and 8.5≤(Mn+Cr)/C≤16; and 
 optionally one or more elements selected from 
 0<Cr≤0.35; 
 0<Cu≤0.20; 
 0<Ni≤0.50; 
 0<Mo≤0.30; 
 0<Nb≤0.10; 
 0<V≤0.10; 
 0<Ti≤0.10; 
 0<B≤0.0030; 
 0<Ca≤0.0050; 
 0<REM≤0.0100, wherein REM is one or more rare earth metals; 
 and the remainder being iron and inevitable impurities; 
 and a microstructure (in vol. %) comprising 
 polygonal ferrite (PF): 0-10; 
 polygonal ferrite (PF)+acicular ferrite (AF)+higher bainitic ferrite (HBF): 5-30; 
 lower bainitic ferrite (LBF): 45-80; 
 retained austenite (RA): 5-20; 
 fresh martensite (M): 0-20. 
 
     
     
         19 . The heat treated cold rolled steel strip according to  claim 18  having at least one of the properties:
 Yield strength (YS) is at least 550 MPa; and/or 
 Tensile strength (TS) is at least 980 MPa; and/or 
 Total elongation (TE) is at least 13%; and/or 
 Hole expansion capacity (HEC) is at least 20%; and/or 
 Bending angle (BA) is at least 80°. 
 
     
     
         20 . A method of heat treating a cold rolled steel strip, which method comprises the steps of:
 a) heating an uncoated cold rolled steel strip from room temperature to a temperature T 1  in the range of 680-740° C. at a single constant heating rate V 1  of 15.0-30.0° C./s;   and further heating the uncoated cold rolled steel strip from the temperature T 1  to a soaking temperature T 2  within a soaking temperature range of (Ac3−20)-(Ac3+20) at a heating rate V 2  of 0.5-4.0° C./s,   a1) then soaking the uncoated cold rolled steel strip at the soaking temperature T 2  within the temperature range of (Ac3−20) to (Ac3+20) for a soaking time t 2  of 30-150 seconds, thereby obtaining a cold rolled steel strip having an austenitic microstructure;   b) cooling the uncoated soaked steel strip resulting from step a1) to a temperature T 4  in the range of Bn-Ms, wherein acicular ferrite is formed during the cooling in a temperature between Bs and Ms; wherein step b) comprises a substep of cooling the soaked steel strip from step a) to a temperature T 3  in the range of 750-600° C., at a cooling rate V 3  of at least 1° C./s, wherein step b) comprises a substep of cooling the soaked steel strip from the temperature T 3 , to T 4  at a cooling rate V 4  of 20.0-60.0° C./s,   and further comprising a substep of cooling the soaked steel strip from a temperature T 3  to T 4  at a cooling rate V 4  of 20.0-60.0° C./s;   c) increasing temperature of the uncoated cooled strip obtained in step b) to a temperature T 5  in a temperature range between Bs and T 4 , to transform austenite to carbide-free LBF and   c1) heat treating the uncoated cooled strip obtained in step c) at the temperature T 5  in a range between the temperature Bs and T 4  for a period of time t 5  of 30-300 seconds;   c2) following heat treatment of step c1) optionally providing the heated treated steel strip with a coating,   d) cooling the heat treated, optionally coated, steel strip to a temperature T 7  in the range of ((Ms-50) to Mf) at a cooling rate of V 7  of 5.0-10.0° C./s and further cooling the heat treated, optionally coated, steel strip to ambient temperature at a cooling rate of V 8  of 5.0-20.0° C./s;   such that the heat treated, optionally coated, steel strip has a microstructure (in vol. %) consisting of:   polygonal ferrite (PF): 0-10;   polygonal ferrite (PF)+acicular ferrite (AF)+carbide-free higher bainitic ferrite (HBF): 5-30;   carbide-free lower bainitic ferrite (LBF): 45-80;   retained austenite (RA): 5-20;   fresh martensite (M): 0-20;   cementite+ferrite less than 5;   wherein carbide-free higher bainitic ferrite (HBF) is present;   wherein the steel strip has a composition (in mass percent) comprising   C: 0.15-0.28;   Mn: 1.70-3.00;   Si: 0.50-2.00;   Al: 0.01-0.60;   P: less than 0.050;   S: less than 0.020;   N: less than 0.0080;   wherein the sum of (Si+Al) is ≥0.60; and   wherein 10C+Mn+Cr≥3.85 and 8.5≤(Mn+Cr)/C≤16; and   optionally one or more elements selected from   0<Cr≤0.35;   0<Cu≤0.20;   0<Ni≤0.50;   0<Mo≤0.30;   0<Nb≤0.10;   0<V≤0.10;   0<Ti≤0.10;   0<B≤0.0030;   0<Ca≤0.0050;   0<REM≤0.0100, wherein REM is one or more rare earth metals;   and the remainder being iron and inevitable impurities.